The modification of polypropylene resins by grafting specific compounds onto the polymer backbone chain in order to produce compositions having improved properties, is previously known.
EP 0,280,454 discloses a method for the grafting of at least two different monomers onto molten hydrocarbon polymer. In working examples the polymer is a homopolymer of propylene, a high density polyethylene or linear low density polyethylene, and the monomers are styrene or maleic anhydride. Contrary to the present method, the process is operated in the absence of an initiator for the grafting reaction and in the substantial absence of antioxidant in the polymer, by the use of an extruder at extrusion conditions.
U.S. Pat. No. 5,079,295 relates to a thermoplastic composition comprising a polyphenylene ether resin, a modified propylene polymer and a rubbery substance. The polypropylene component is grafted with a styrene-based monomer and an unsaturated carboxylic acid or its derivative. A number of grafting processes are described. In the working examples 100 parts by weight of polypropylene are mixed with one part of maleic anhydride (MAH), one part of styrene and one part of peroxide in a mixer, and subsequently extruded at 220.degree. C. In another embodiment in an autoclave a dispersion of polypropylene, styrene and MAH in water is allowed to react at 120.degree. C. In further embodiments the MAH of said dispersion is replaced by glycidyl methacrylate or glycidyl acrylate.
U.S. patent application Ser. No. 924,786 relates to polypropylene compositions grafted with glycidyl acrylate or glycidyl methacrylate (GMA) using an organic peroxide as a radical-generating agent, performed by kneading the melted mixture, preferably in an inert atmosphere.
In a grafting process, the high molecular weight of the virgin polypropylene resin should be maintained to retain its desired physical and mechanical properties. If only one type of monomer, such as a monomeric allylepoxy compound, is used in a polypropylene grafting method, the achieved grafting efficiency is poor and accompanied by a heavy degradation of the polymer by chain scission in the .beta.-position and simultaneous homopolymerization of the added monomer. Such a degradation of the polypropylene will inevitably result in polypropylene compositions having a reduced molecular weight, and consequently articles manufactured therefrom will have mechanical properties inferior to those of articles made from virgin polypropylene resins. The problem of chain scission would be avoided if the grafting could be performed in the presence of a compound having a stabilizing effect on the intermediate polymeric radicals created during the grafting process. The composition would then maintain its high molecular weight and its highly valued physical properties.